In two-stroke engines, the reciprocal movement of a piston inside a cylinder opens and closes the scavenge and exhaust ports. As the piston continues to move up when the ports are closed, the gases above the piston are compressed. This compression is desirable for the combustion of an air-fuel mixture in the cylinder. However during engine start-up, before a first combustion event can occur in the cylinder, having to compress the gases makes turning the crankshaft connected to the piston harder. As a result, engines having electrical starter motors need to have starter motors that are stronger than if no compression occurred. In engines that are started using manual crank starters, the users need to pull harder on the crank than if no compression occurred.
One solution consists in providing the engine with what is commonly known as a decompression system. Decompression systems provide a passage, called a decompression passage, through which gases above the piston can escape when the scavenge and exhaust ports are closed as the piston moves up, thereby reducing the amount of compression that occurs, thus facilitating engine start-up.
However, in some of these systems, the decompression passage is open during some or all operating conditions of the engine following start-up. As a result, the operational efficiency of the engine is reduced.
One solution to this problem that other decompression systems have used consists in providing a valve for opening the decompression passage during engine start-up and for closing the decompression passage after the engine has started. However, the addition of such a valve adds cost and complexity to the engine.
There is therefore a desire for a decompression system that does not substantially add cost and/or complexity to the engine.
In order to ensure that two-stroke engines have a high power capacity at high speeds, a high volumetric efficiency is required and the charge losses must be minimized. This can be accomplished by an early and therefore higher opening of the exhaust passage into the cylinder. In order to obtain maximum power capacity of the engine at high speeds, the adjustment of the exhaust port involves, in the medium speed range, not only an appreciable decrease of the useful stroke, but also a large increase of the charge losses. As a result, the torque decreases and the specific fuel consumption increases greatly. A higher torque in conjunction with lower fuel consumption can be obtained, at lower engine speeds, only if the opening of the exhaust port happens later in the down stroke of the piston. This means that the exhaust port must be at a lower position than it is at high engine speeds.
For this purpose it is known to provide a valve in the exhaust port which is movable between a full flow position and a flow restricting position. When in the flow restricting position, the end of the valve is substantially flush with the peripheral surface of the cylinder bore. In this flow restricting position, the exhaust port is effectively lowered in relation to the down stroke of the piston. The valve is adjustable to vary the relative height of the exhaust port as is required by the given operating conditions of the engine.
During operation, although two-stroke engines are becoming cleaner, some of the fuel and oil does not burn completely. Some of the unburnt fuel and oil, known as coke, sticks to the exhaust valve. The coke on the exhaust valve can cause the valve to stick to the walls of the cylinder block, thereby preventing its proper movement.
To prevent the exhaust valves from sticking due to the coke build up, one solution consists in cycling the exhaust valves rapidly through it various positions. This is typically done at engine start-up or shut-down where the effect on efficiency of the engine is minimal. However, coking can occur during long operation of the engine.
There is therefore a desire for a method for cleaning an exhaust valve during operation of the engine.